An electric power steering apparatus (EPS) which provides a steering mechanism of a vehicle with an assist torque by means of a rotational torque of a motor, applies a driving force of the motor as a steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus (EPS) performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Td of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and the motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command on the basis of the steering torque Td detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value. A steering angle sensor 14 is not indispensable and may not be provided.
A controller area network (CAN) 50 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed V from the CAN 50. Further, a Non-CAN 51 is also possible to connect to the control unit 30, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 30 mainly comprises a central processing unit (CPU) (including a micro controller unit (MCU) and a micro processing unit (MPU)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Td detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 (or from the CAN 50) are inputted into a current command value calculating section 31 which calculates the current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1, based on the steering torque Td and the vehicle speed V with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, and the current command value Irefm whose maximum current is limited is inputted into a subtracting section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated at the subtracting section 32B, and the deviation I is inputted into a proportional-integral-control (PI-control) section 35 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved at the PI-control section 35, is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed-back to the subtracting section 32B. An FET is used as a driving device at the inverter 37, and the inverter 37 is constituted by a bridge circuit of the FET.
A compensation signal CM from a compensation signal generating section 34 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and a convergence, an inertia characteristic, and the like are improved. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 to an inertia 342 at an adding section 344. The adding result is further added with a convergence 341 at an adding section 345. The adding result at the adding section 345 is treated as the compensation signal CM.
In such an electric power steering apparatus, friction due to the reduction gears and the rack-and-pinion is large, and an equivalent inertia moment around the steering shaft due to the motor in order to generate the assist torque is large. Thereby, in a low vehicle speed range that the self-aligning torque (SAT) is small, the handle-returning is not adequate since the friction is larger than the SAT. Since the steering angle is not returned to a neutral position by only the SAT in a straight running state, it is necessary to return the steering angle to the neutral position by means of a steering intervention by a driver, and this is a burden on the driver.
On the other hand, since the SAT is also large in a high vehicle speed range that the SAT is large, the steering angular velocity of the handle-returning tends to be fast in comparison with the low vehicle speed due to the large SAT. However, an inertia torque is also large since the inertia moment is large. Consequently, the handle is not converged at the neutral position of the steering angle and is overshot, and therefore the vehicle characteristics are felt unstable.
Thus, it is necessary to assist the handle-returning in the low vehicle speed and to enhance the convergence in order to stabilize the vehicle characteristics in the high vehicle speed. In order to achieve the above requirements, various control methods, which perform an appropriate assist when returning the handle, are proposed. In those handle-returning controls, the electric power steering apparatus as a prior art whose object is to perform the smooth handle-returning control even in the steering intervention by the driver is disclosed in Japanese Patent No. 4685557 B2 (Patent Document 1).
In the apparatus of Patent Document 1, a controller constituted so as to follow the target steering angular velocity calculates the target steering angular velocity by performing a correction, for a base target steering angular velocity, by using a multiplication and an addition based on the vehicle speed and the steering torque. In the steering intervention by the driver, uncomfortable feeling for the driver when the driver steers the handle, is reduced by correcting the target steering angular velocity to a direction which the steering torque is applied.